1. Field of Invention
This invention relates to waveguide attenuators and phase shifters and, more particularly, to such attenuators and phase shifters which are optically activated.
2. Description of the Prior Art
There are applications which utilize waveguides to pass microwave and millimeter wave signals where it is desired to phase shift or attenuate or, in the extreme, switch off the microwave or millimeter wave signal. It is known to utilize, as a switch, a waveguide with a light responsive semiconductor in the waveguide cavity being orthogonal to the direction of signal propagation. When light from a light source traveling in the waveguide parallel to the direction of signal propagation in the waveguide strikes the semiconductor, it changes from a dielectric to a conductor blocking the signal which would otherwise pass through the waveguide. A problem with such a switch is the complexity of the structure due to the light location and the cost of construction.
It is also known that dielectric mode devices could function as attenuators and phase shifters as disclosed in "Optical Control of Millimeter-Wave Propagation in Dielectric Waveguides" IEEE Journal of Ouantum Electronics, Vol. QE-16, No. 3, March 1980, pp. 277-287, by Chi H. Lee, P. S. Mak and A. P. DeFonzo. A dielectric mode device consists of a rectangular semiconductor waveguide with tapered ends to allow efficient transition to and from a conventional metallic waveguide. Optical control is realized when the broad wall of the semiconductor guide is illuminated by laser light creating plasma. The effect of the plasma occupied region is to introduce a layer whose index of refraction at millimeter waves is different from the remaining volume of the bulk semiconductor, thus providing phase shifting and attenuation effect. In the prior art, however, the isolation between phase shifting and attenuation is difficult. In addition, the arrangement calls for a high power continuous wave laser, making the device bulky and expensive.
The design and fabrication of electronically controlled phase shifters and attenuators at millimeter wave is also know using bulk semiconductors in guided wave structures as disclosed in "Millimeter-Wave Phase Shifter," RCA Review, Vol. 34, Sept. 1973, pp. 489-505, by B. J. Levin and G. G. Weidner. Phase shift/attenuation is produced by the electronic modulation of the width of a rectangular waveguide. The change in effective width of the waveguide is accomplished by means of a PIN diode that is literally distributed along the small side wall of the waveguide. In the embodiment, the interaction (lack of isolation) between the power supply driving the PIN device and the propagating millimeter wave appears to limit the useful bandwidth of this device.